Lyme disease (LD) caused by the Ixodes tick-borne spirochete Borrelia burgdorferi (Bb), is the most common vector-borne disease in the United States. Despite public health preventive measures, the annual confirmed case incidence has risen to over 30,000, the vast majority of which occur in the Northeast. Disseminated infection causes disease in the skin, heart, joints and nervous system and can be difficult to treat. Antibiotics achieve clinical cure in the majority of people when administered in early stages of infection. Unexplained symptoms can linger in up to 25% of patients, however, which may or may not be due to persistence of active infection. Current serologic tests that are the mainstay of LD diagnostics cannot be used to assess response to therapy and cannot distinguish previous exposure to Bb from active infection. Patients thus often receive unnecessary and potentially dangerous long-term antibiotic treatments or other unconventional and unproven treatments. This application seeks to improve the current situation by developing a new diagnostic serologic assay based on a novel panel of Bb antigens to which antibody levels decline rapidly after successful elimination of infection by antibiotics. We have shown using a mouse model of Lyme borreliosis that successful elimination of infection with antibiotic treatment correlates with declining responses to panel antigens, and that failure of antibiotics to completely eliminate infection is associated with maintenance of elevated responses to panel antigens. We also demonstrated in a retrospective study of banked human C6 peptide positive Lyme sera that most human samples react with 4 or more panel antigens. We will conduct a prospective study of a large number of newly diagnosed LD patients undergoing antibiotic therapy. We will monitor the decline of antibody levels to individual antigens and select from our panel a group of antigens for which this decline best correlates with resolution of symptoms after antibiotic treatment. We will also determine if patients with persistent symptoms at 3 months after treatment maintain high levels of antibodies to panel antigens up to one year later. As a strategy that complements current serologic tests, this assay will provide additional tools for clinicians to objectively base decisions on additional courses of antibiotics for symptoms that may be due to incompletely treated or new Bb infections.